Measuring depth to groundwater in monitoring wells is one of the most fundamental observations in hydrogeology. Using interpolation (and extrapolation) maps of depth to water table are created, which can have significant errors due to sparse measurements and also subtle changes in low lying topography. In semi-arid regions, like the Northern Adelaide Plains (NAP), South Australia, assessing zones with shallow groundwater depths is crucial; especially as extension of irrigation with treated sewage plant effluent could result in rising water tables. However, limited availability of and/or high costs of installing piezometers means that we still lack methods of fast and reliable characterization of shallow groundwater. An obvious interesting opportunity is to use non-intrusive hydrogeophysical sensing techniques.
We have collected geophysical data sets at three study sites within the NAP. Techniques evaluated include: a frequency domain, shallow terrain conductivity meter (CMD); a fast-sampling time domain electromagnetics system (TEM); a resistivity system (ERT); and a shallow reflection seismic system. Additionally, to provide further information on soil variability, as well as groundwater depth and quality, 47 geoprobe boreholes were drilled, 20 of them were logged using a shallow borehole nuclear magnetic resonance (NMR) system.
Results suggest that a) the main river systems, as well as much more subtle ephemeral streams, appear to be important recharge zones; b) the seismic survey provide useful information about depth to groundwater; c) geophysical techniques that measure the ground conductivity (i.e. TEM, ERT, CMD) can determine the depth to groundwater when there is a sufficient contrast between the conductivity of the groundwater and the background soils; d) borehole NMR results of water content and water boundness were generally consistent with those obtained from samples evaluated in the laboratory; and e) NMR has the potential to identify the presence of heavy clays that could impede infiltration.